CANADIAN TREE PORCUPINE 71 



of tree porcupines have apparently advanced northward along 

 the isthmus of Central America and have reached Mexico 

 within quite recent times. There are no grounds, however, 

 for the supposition that Erethizon is a modified Coendu. 

 When North and South America became joined by the exist- 

 ing isthlnus of Central America in Pliocene times, Erethizon 

 no doubt was already an inhabitant of the northern continent. 

 Since the two genera belong to the same family Coendidae, 

 which differs fundamentally from the Old World family 

 Hystricidae, we must assume that long anterior to the Plio- 

 cene Period North and South America, or such portions of 

 those continents which then existed, had already been united 

 and then become disconnected again. These theories 3xe 

 not founded on zoogeographical data alone. Professor 

 Osborn * urges on palaeontological grounds that North and 

 South America were joined in Cretaceous and perhaps in 

 early Tertiary times, and tihen separated again until the 

 Pliocene. 



All this will be discussed in detail when we come to deal 

 with Central America. I only mention the matter now 

 because in the Santa Cruz beds of Patagonia the skeleton of 

 a peculiar tree porcupine has been discovered and placed 

 by Dr. Ameghino into the new genus " Steiromys." More 

 recently. Professor W. B. Scottf re-examined these Steiromys 

 remains, and noted the remarkable fact that they are more 

 like the recent Erethizon of North America than any of the 

 modern South American tree porcupines. He even argues 

 that Steiromys is the direct ancestor of Erethizon. 



As this implies a former land connection between Pata- 

 gonia and North America, independently of the remainder 

 of South America, it largely aided me in the theoretical 

 construction of an ancient land bridge between south-western 

 North America and Chile. J It is on this land bridge, I think, 

 that the ancestor of the North American Erethizon wandered 

 northward from Patagonia in eai'ly Tertiary times. All this 

 will be more fully explained later on. 



* Osborn, H. F., "Progress in Mammalian Palaeontology," p. 99. 

 t Scott, W. B., " Eeport of Princeton Expedition," V., pp. 413—417. 

 I Scharff, E. F., " Early Tertiary Land-Connection," p. 521, 



